Method for forming an image sensing device

ABSTRACT

A method for forming an image sensing device is disclosed. An epitaxy layer having the first conductivity type is formed on a substrate, wherein the epitaxy layer comprises a first pixel area corresponding to a first incident light, a second pixel area corresponding to a second incident light, and a third pixel area corresponding to a third incident light. A first deep well is formed in a lower portion of the epitaxy layer for reducing pixel-to-pixel talk of the image sensing device. A second deep well is formed in a lower portion of the epitaxy layer.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a Divisional of pending U.S. patent application Ser.No. 12/898,419, filed Oct. 5, 2010 and entitled “IMAGE SENSING DEVICEAND FABRICATION THEREOF”.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an image sensing device and to a methodof fabricating the same, and more particularly, to an image sensingdevice having reduced pixel-to-pixel crosstalk and quantum efficiency,and a method of fabricating the same.

2. Description of the Related Art

An image sensor converts optical information into electrical signals.Recently, with advanced development of the computer and communicationsindustries, there has also been an increasing demand for morehighly-efficient image sensors for various fields such as digitalcameras, camcorders, personal communication systems, game devices,surveillance cameras, micro-cameras for medical use, robots, and so on.

A unit pixel in an image sensor converts incident light into anelectrical signal, and integrates charges corresponding to the amount oflight at a photoelectric converter. In addition, a unit pixel of animage sensor reproduces an image signal through a readout operation.However, the incident light may form charges that are not integrated atthe photoelectric converter of the unit pixel. For example, in a CMOSimage sensor, charges may be moved to and integrated at a photoelectricconverter of an adjacent pixel, causing what is known as pixel-to-pixelcrosstalk.

Referring to FIG. 1, pixel-to-pixel crosstalk may be divided into thefollowing categories: (a) optical crosstalk A, which occurs when areflected light 6 is transmitted to a photoelectric converter 2 of aunit pixel adjacent to a relevant unit pixel, a reflected light 6 isformed by reflection from a top or side of metal wirings M1, M2, and M3,or a refractive light is formed by refraction at a non-uniform layer orat a multilayered structure including interlayer insulating layershaving different refractive indexes; and (b) electrical crosstalk B,which occurs when charges generated at a lower or side portion of aphotoelectric converter 2 of a relevant unit pixel are delivered to thephotoelectric converter 2 of an adjacent unit pixel via along-wavelength incident light 7.

For a black and white image sensor, when crosstalk occurs, resolutiondecreases causing image distortion. Meanwhile, for a color image sensorusing an RGB color filter array (CFA), the probability for crosstalk tooccur due to red light having a long wavelength is high, which may causepoor tint quality. Moreover, crosstalk may cause blooming effect inwhich adjacent pixels for an image are blurred.

BRIEF SUMMARY OF INVENTION

A method for forming an image sensing device is disclosed. A substratehaving a first conductivity type is provided. An epitaxy layer havingthe first conductivity type is formed on the substrate, wherein theepitaxy layer comprises a first pixel area corresponding to a firstincident light, a second pixel area corresponding to a second incidentlight, and a third pixel area corresponding to a third incident light,wherein wavelength of the first incident light is longer than that ofthe second incident light and the wavelength of the second incidentlight is longer than that of the third incident light. A first deep wellis formed in a lower portion of the epitaxy layer for reducingpixel-to-pixel talk of the image sensing device by implantation using amask, wherein the mask has a cover corresponding to the first pixel areaand a portion of the mask corresponding to the second pixel area and thethird pixel area are exposed, such that the implanting process can formthe first deep well at least in the second pixel area and the thirdpixel area of the epitaxy layer, and at least a portion of the firstpixel area does not comprise the first deep well. A second deep well isformed in a lower portion of the epitaxy layer, wherein the second deepwell is at least disposed in the first, second and third pixel area, andthe portion of the first pixel area not comprising the first deep wellcomprises the second deep well. A photodiode is formed in an upperportion of the epitaxy layer.

BRIEF DESCRIPTION OF DRAWINGS

The invention can be more fully understood by reading the subsequentdetailed description and examples with references made to theaccompanying drawings, wherein:

FIG. 1 shows a cross section of a conventional image sensing device;

FIG. 2 is a cross section of a pixel of an image sensor having someissues found by the inventor;

FIG. 3 shows a curve diagram with relative quantum efficiency (QE) as afunction of wavelength of R, G, B color light;

FIG. 4A shows a cross section of a green pixel of an image sensingdevice of an embodiment of the invention;

FIG. 4B shows a cross section of a blue pixel of an image sensing deviceof an embodiment of the invention;

FIG. 4C shows a cross section of a red pixel of an image sensing deviceof an embodiment of the invention;

FIG. 5 shows a novel pixel buried well mask of an embodiment of theinvention;

FIG. 6 shows a cross section of a red pixel of an image sensing deviceof another embodiment of the invention;

FIG. 7 shows a cross section of a red pixel of an image sensing deviceof further another embodiment of the invention;

FIG. 8A shows a cross section of a green pixel of an image sensingdevice of another embodiment of the invention;

FIG. 8B shows a cross section of a blue pixel of an image sensing deviceof another embodiment of the invention;

FIG. 8C shows a cross section of a red pixel of an image sensing deviceof another embodiment of the invention;

FIG. 9A shows a cross section of a green pixel of an image sensingdevice of further another embodiment of the invention;

FIG. 9B shows a cross section of a blue pixel of an image sensing deviceof further another embodiment of the invention;

FIG. 9C shows a cross section of a red pixel of an image sensing deviceof further another embodiment of the invention;

FIG. 10A shows a cross section of a green pixel of an image sensingdevice of yet another embodiment of the invention;

FIG. 10B shows a cross section of a blue pixel of an image sensingdevice of yet another embodiment of the invention; and

FIG. 10C shows a cross section of a red pixel of an image sensing deviceof yet another embodiment of the invention.

DETAILED DESCRIPTION OF INVENTION

FIG. 2 is a cross section of a pixel of an image sensor having someissues found by the inventor. Following a description of FIG. 2,problems found in the fabrication method thereof will be highlighted.Referring to FIG. 2, a p-type epitaxy layer 204 is grown on a p-typesubstrate 202. A photodiode 206 comprising an n well 208 and p-type pinlayer 210 is disposed in the p-type epitaxy layer 204 with one sideneighboring a transfer gate 212. The photo diode 206 of the pixel isisolated from other pixels by a shallow trench isolation 214, isolatingp well 216 thereunder. In order to suppress pixel-to-pixel cross talk,the p-type epitaxy layer 204 is further implanted to form a pixel deep pwell (DPW) 218 and a pixel deep n well (DNW) 220 under the photodiode206.

FIG. 3 is a curve diagram with relative quantum efficiency (QE) as afunction of wavelength of R, G, B color light. In FIG. 3, it is shownthat pixel-to-pixel cross talk is suppressed, but the image sensor haslow quantum efficiency for long-wave light (red light). The pixel deep pwell 218 and pixel deep n well 220 form a potential barrier so thatcharges generated deep in the lower area of the epitaxy layer 204 areprevented from flowing into the photodiodes of adjacent pixels. Thus,the pixel deep p well 218 and pixel deep n well 220 hinderspixel-to-pixel crosstalk caused by randomly drifting charges.

For example, the deep p well and the deep n well may have a maximumconcentration at a depth of about 3-10 μm from a surface of the epitaxylayer and may have a thickness of about 1-5 μm. Here, the depth of about3-10 μm is substantially the same as an absorption length of red or nearinfrared region light. When the depth of the deep p well 218 and thedeep n well 220 from the surface of the epitaxy layer 204 becomesshallow, a diffusion prevention effect increases, and therefore,crosstalk decreases. However, since the photo diode 206 also becomeshallow, sensitivity with respect to incident light having a longwavelength (e.g., a red wavelength) that has a high photoelectricconversion rate at a deep region may decrease. Accordingly, although themethod described above reduce pixel-to-pixel crosstalk, quantumefficiency is sacrificed for light propagating at longer wavelengths,especially for red light. In addition, an R, G, B color quantumefficiency gap is generated, which hinders image signal processing.

FIG. 4A˜FIG. 4C are cross sections of a green pixel, a blue pixel and ared pixel of an image sensing device which can suppress cross talk,while maintaining good quantum efficiency of an embodiment of theinvention. FIG. 4A shows a green pixel structure, FIG. 4B shows a bluepixel structure, and FIG. 4C shows a red pixel structure. In FIG. 4A,FIG. 4B and FIG. 4C, a substrate 402 is provided. The substrate 402 canbe a p-type substrate, for example comprising boron. A p-type epitaxylayer 404 is grown on the substrate 402. The p-type epitaxy layer 404can be doped with boron and can be formed on the substrate 402 bychemical vapor deposition (CVD) or molecular beam epitaxy (MBE). In anembodiment, the p-type epitaxy layer 404 can be about 3-10 μm thick. Thep-type epitaxy layer 404 is implanted using a novel pixel buried wellmask 502 (the mask as FIG. 5) to form a deep p well 406 and a deep nwell 408 under the deep p well 406. Note that the mask 502 in FIG. 5 hascovers corresponding to a red pixel area 504, wherein a green pixel area506 and a blue pixel area 508 are exposed. Therefore, the ionimplantation process can only dope the epitaxy layer 404 in the greenpixel area 506 and the blue pixel area 508 to form a deep p well 406 anda deep n well 408, and a least a portion of the red pixel area 504 isnot doped. In other words, the image sensor of the embodiment has a deepp well 406 and a deep n well 408 in the green pixel area 506 and bluepixel area 508, but at least a portion of the red pixel area 504 doesnot comprise the deep p well and the deep n well. The size of the area410 without the deep p well and the deep n well in the red pixel 504 canbe dependant upon product specifications or process conditions. In anembodiment, as shown in FIG. 6, the size of the area 410 a without thedeep p well and the deep n well is less than the pixel area. In analternative embodiment, as shown in FIG. 7, the size of the area 410 bwithout the deep p well and the deep n well is larger than the pixelarea.

Referring back to FIG. 4A, FIG. 4B and FIG. 4C, an isolation structure412 is formed in the epitaxy layer 404. The isolation structure 412 canbe a shallow trench isolation (STI) or a field oxide. Preferably, theisolation structure 412 is a shallow trench isolation (STI) structure.An isolating p well 414 is formed under the isolation structure 412 byion implantation. A transfer gate 416 is formed on a surface of theepitaxy layer 404. In an embodiment, the transfer gate 416 can comprisea gate dielectric layer 418, a gate conductive layer 420 on the gatedielectric layer 418, and spacers 422 on sidewalls of the gateconductive layer 420. The transfer gate can be formed by the followingsteps. First, a gate dielectric layer 418 is formed on the epitaxy layer404. Next, a gate conductive layer 420 is formed on the gate dielectriclayer 418. Following, the gate conductive layer 418 and the gatedielectric layer 420 are patterned by a lithography and etching process.Next, a spacer layer (not shown) is deposited on the gate conductivelayer 420 and the epitaxy layer 404. Following, the spacer layer isetched to form spacers 422 on sidewalls of the gate conductive layer420. The epitaxy layer 404 is implanted to form an n well 424 and ap-type pin layer 426, thus forming a photodiode 428.

The described image sensing device of the invention addresses the issueconcerning decreased sensitivity with respect to incident light havinglong wavelengths (e.g., a red wavelength) that has a high photoelectricconversion rate at deep region. As well, the described image sensingdevice and fabrication method thereof of the invention, improves quantumefficiency with good R, G, and B color balance, when compared toconventional methods, while reducing crosstalk, without adding extraprocess steps and negatively effecting pixel performance.

The embodiments described disclose n-type image sensing devices. Theinvention, however, is not limited thereto. A p-type image sensingdevice may also utilize the fabrication method of the invention. FIG.8A˜FIG. 8C are cross sections of a green pixel, a blue pixel and a redpixel, respectively, showing reduced cross talk, while maintaining goodquantum efficiency of a p-type image sensor of an embodiment of theinvention. FIG. 8A shows a green pixel structure, FIG. 8B a shows bluepixel structure, and FIG. 8C shows a red pixel structure. In FIG. 8A,FIG. 8B and FIG. 8C, a substrate 802 is provided. The substrate 802 canbe an n-type substrate, for example comprising phosphorous or arsenic.An n-type epitaxy layer 804 is grown on the substrate 802. In anembodiment, the n-type epitaxy layer 804 can be about 3-10 μm thick. Then-type epitaxy layer 804 is implanted using a novel pixel buried wellmask to form a deep n well 806 and a deep p well 808 under the deep nwell 806. Note that the novel pixel buried well mask has coverscorresponding to red pixel areas 810 but the green pixel areas 812 andthe blue pixel areas 814 are exposed. Therefore, the ion implantationprocess can dose the epitaxy layer 804 in the green pixel area 812 andthe blue pixel layer 814 to form a deep n well 806 and a deep p well808, but a least a portion of the red pixel area 810 is not doped. Inother words, the image sensor of the embodiment has a deep n well 806and a deep p well 808 in the green pixel area 812 and blue pixel area814, but at least a portion of the red pixel area 810 does not comprisethe deep n well and the deep p well. The size of the area 816 withoutthe deep n well 806 and the deep p well 808 in the red pixel 810 can bedependant upon product specification or process conditions. In anembodiment, the size of the area 816 without the deep n well and thedeep p well is less than the pixel area 810. In an alternativeembodiment, the size of the area 816 without the deep n well and thedeep p well is larger than the pixel area 810.

As shown in FIG. 8A, FIG. 8B and FIG. 8C, an isolation structure 818 isformed in the epitaxy layer 804. The isolation structure 818 can be ashallow trench isolation (STI) or a field oxide. Preferably, theisolation structure 818 is a shallow trench isolation (STI). Anisolating n well 820 is formed under the isolation structure 818 by ionimplantation. A transfer gate 822 is formed on the epitaxy layer 804. Inan embodiment, the transfer gate 822 can comprise a gate dielectriclayer 824, a gate conductive layer 826 on the gate dielectric layer 824,and spacers 828 on sidewalls of the gate conductive layer 826. Theepitaxy layer 824 is implanted to form a p well 830 and an n-type pinlayer 832, thus forming a photodiode 834.

FIG. 9A˜FIG. 9C are cross sections of a green pixel, a blue pixel and ared pixel, respectively, of an image sensing device which can suppresscrosstalk, while maintaining good quantum efficiency of anotherembodiment of the invention. FIG. 9A shows a green pixel structure, FIG.9B shows a blue pixel structure, and FIG. 9C shows a red pixelstructure. The difference between the embodiment in FIG. 4A˜FIG. 4C andFIG. 9A˜FIG. 9C is that a portion of the red pixel area does not have adeep p well and a deep n well in the embodiment in FIG. 4A˜FIG. 4C,while a portion of the red pixel area only does not have deep p well buthas a deep n well in the embodiment in FIG. 9A˜FIG. 9C.

In FIG. 9A, FIG. 9B and FIG. 9C, a substrate 902 is provided. Thesubstrate 902 can be a p-type substrate. A p-type epitaxy layer 904 isgrown on the substrate 902. In an embodiment, the p-type epitaxy layer904 can be about 3-10 μm thick. The p-type epitaxy layer 904 isimplanted using a novel pixel buried well mask to form a deep p well906. Note that the novel pixel buried well mask has covers correspondingto red pixel areas 908 but the green pixel areas 910 and the blue pixelareas 912 are exposed. Therefore, the ion implantation process can dosethe epitaxy layer 904 in the green pixel area 910 and the blue pixellayer 912 to form a deep p well 906, wherein a least a portion of thered pixel area 908 is not doped to form the deep p well 906. Inaddition, the p-type epitaxy layer 904 is further implanted to form adeep n well 914 under the deep p well 906. Note that the ionimplantation process doses the epitaxy layer 904 in the red, green andblue pixel area 908, 910, 912 to form deep p wells 906. As well, theimage sensor in the embodiment does not comprise deep p well, butcomprises deep n well 914 in the red pixel area 908. Note that both thedeep p well 906 and the deep n well 914 are formed in the green and bluepixel areas.

In FIG. 9A, FIG. 9B and FIG. 9C, an isolation structure 914, such as ashallow trench isolation (STI), is formed in the epitaxy layer 904. Anisolating p well 916 is formed under the isolation structure 914 by ionimplantation. A transfer gate 918 is formed on the epitaxy layer 904.The epitaxy layer is implanted to form an n well 920 and a p-type pinlayer 922, thus forming a photodiode 924.

FIG. 10A˜FIG. 10C are cross sections of a green pixel, a blue pixel anda red pixel of an image sensing device which can suppress crosstalk,while maintaining good quantum efficiency of yet another embodiment ofthe invention. FIG. 10A shows a green pixel structure, FIG. 10B shows ablue pixel structure, and FIG. 10C shows a red pixel structure. Thedifference between the embodiment in FIG. 4A˜FIG. 4C and FIG. 10A˜FIG.10C is that a portion of the red pixel area does not have deep p welland deep n well in the embodiment in FIG. 4A˜FIG. 4C, while a portion ofthe red pixel area only does not have deep n well but has a deep p wellin the embodiment in FIG. 10A˜FIG. 10C.

In FIG. 10A, FIG. 10B and FIG. 10C, a substrate 1002 is provided. Thesubstrate 1002 can be a p-type substrate. A p-type epitaxy layer 1004 isgrown on the substrate 1002. In an embodiment, the p-type epitaxy layer1004 can be about 3-10 μm thick. The p-type epitaxy layer 1004 isimplanted to form a deep p well 1006. Note that the deep p well 1006 isformed in the red pixel area 1008, the green pixel area 1010 and theblue pixel area 1012. The p-type epitaxy layer 1004 is implanted using anovel pixel buried well mask to form a deep n well 1014 under the deep pwell 1006. Note that the novel pixel buried well mask has coverscorresponding to red pixel areas 1008 but the green pixel areas 1010 andthe blue pixel areas 1012 are exposed. Therefore, the ion implantationprocess can dose the epitaxy layer 1004 in the green pixel area 1010 andthe blue pixel area 1012 to form a deep n well 1014, wherein at least aportion of the red pixel area 1008 does not comprises deep n well. Aswell, the image sensor in the embodiment does not comprise deep n wellbut comprises deep p well 1016 in the red pixel area. Note that both thedeep p well 1006 and the deep n well 1014 are formed in the green pixelarea 1010 and blue pixel area 1012.

In FIG. 10A, FIG. 10B and FIG. 10C, an isolation structure 1014 such asa shallow trench isolation (STI), is formed in the epitaxy layer 1004.An isolating p well 1016 is formed under the isolation structure 1014 byion implantation. A transfer gate 1018 is formed on the epitaxy layer1004. The epitaxy layer 1004 is implanted to form an n well 1020 and ap-type pin layer 1022, thus forming a photodiode 1024.

The described image sensing device and fabrication methods thereof ofthe invention improves quantum efficiency with good R, G, and B colorbalance, while reducing crosstalk, without adding extra process stepsand negatively affecting pixel performance.

While the invention has been described by way of example and in terms ofthe preferred embodiments, it is to be understood that the invention isnot limited to the disclosed embodiments. It is intended to covervarious modifications and similar arrangements (as would be apparent tothose skilled in the art). Therefore, the scope of the appended claimsshould be accorded the broadest interpretation so as to encompass allsuch modifications and similar arrangements.

What is claimed is:
 1. A method for forming an image sensing device,comprising: providing a substrate having a first conductivity type;forming an epitaxy layer having the first conductivity type on thesubstrate, wherein the epitaxy layer comprises a first pixel areacorresponding to a first incident light, a second pixel areacorresponding to a second incident light, and a third pixel areacorresponding to a third incident light, and wavelength of the firstincident light is longer than that of the second incident light andwavelength of the second incident light is longer than that of the thirdincident light; forming a first deep well in a lower portion of theepitaxy layer for reducing pixel-to-pixel talk of the image sensingdevice by implantation using a mask, wherein the mask has a covercorresponding to the first pixel area and a portion of the maskcorresponding to the second pixel area and the third pixel area areexposed, such that the implanting process can form the first deep wellat least in the second pixel area and the third pixel area of theepitaxy layer, and at least a portion of the first pixel area does notcomprise the first deep well; and forming a second deep well in a lowerportion of the epitaxy layer, wherein the second deep well is at leastdisposed in the first, second and third pixel area, and the portion ofthe first pixel area not comprising the first deep well comprises thesecond deep well; and forming a photodiode in an upper portion of theepitaxy layer.
 2. The method for forming an image sensing device asclaimed in claim 1, wherein the first incident light is red, the secondincident light is green and the third incident light is blue.
 3. Themethod for forming an image sensing device as claimed in claim 1,wherein the second deep well is under the first deep well, and the firstdeep well is the first conductivity type and the second deep well is asecond conductivity type.
 4. The method for forming an image sensingdevice as claimed in claim 3, wherein the first conductivity type is ptype and the second conductivity type is n type.
 5. The method forforming an image sensing device as claimed in claim 1, wherein thesecond deep well is above the first deep well, and the first deep wellis a second conductivity type and the second deep well is the firstconductivity type.
 6. The method for forming an image sensing device asclaimed in claim 5, wherein the first conductivity type is p type andthe second conductivity type is n type.
 7. The method for forming animage sensing device as claimed in claim 1, wherein the step of formingthe photodiode comprises forming a well of a second conductivity typeand a pin layer of the first conductivity type in the epitaxy layer. 8.The method for forming an image sensing device as claimed as claimed inclaim 1, wherein the size of the area of not comprising the first deepwell is greater than the size of the first pixel area.
 9. The method forforming an image sensing device as claimed as claimed in claim 1,wherein the size of the area of not comprising the first deep well issmaller than the size of the first pixel area.
 10. The method forforming an image sensing device as claimed as claimed in claim 1,further comprising an isolation structure and an isolating well of thefirst conductivity type under the isolation structure to make the firstpixel area, the second pixel area and the third pixel area isolated fromeach other.